Re-add download_manager module in utils

[lhoestq]

#4384 moved datasets.utils.download_manager to datasets.download.download_manager

This breaks evaluate which imports DownloadMode from datasets.utils.download_manager

This PR re-adds datasets.utils.download_manager without circular imports.

We could also show a message that says that accessing it is deprecated, but I think we can do this in a subsequent PR, and just focus on doing a patch release for now

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[albertvillanova]

Thanks for the fix.

I'm wondering how this fixes backward compatibility...

Executing this code:

from datasets.utils.download_manager import DownloadMode

we will have

DownloadMode = None

If afterwards we use something like:

if download_mode == DownloadMode.FORCE_REDOWNLOAD

that will raise an exception.

[lhoestq]

It works fine on my side:

>>> from datasets.utils.download_manager import DownloadMode
>>> DownloadMode is not None
True

[lhoestq]

As reported in huggingface/evaluate#143

from datasets.utils import DownloadConfig

is also missing, I'm re-adding it

[lhoestq]

Took the liberty of merging this one, to do a patch release soon. If we think of a better approach we can improve it later